The growth of EBV-infected B cells in vitro was inhibited by autologous T lymphocytes. Light lymphocytes acted promptly and their effect is independent of proliferation, while the heavy, small T cells proliferated and acted only in later stages of the mixed cultures. We found that their effect is abrogated by irradiation or treatment with cyclosporin. We generated cytotoxic T cell clones that lysed autologous LCL. They carried the OKT8 or the OKT4 marker. One OKT4 culture lysed in addition to the LCL, autologous Protein A-induced B blasts. This effect was specific since allogeneic blasts were not lysed. We intend now to analyze the distinction between the recognition of EBV-induced surface antigen and the response to B blasts. The generation of anti-K562, anti-Daudi, and anti-autologous PHA blast cytotoxicity was studied in mixed lymphocyte cultures. The precursors of the latter two and the alloreactive potential were E-rosetting T cells. The anti-K562 effect was stronger in the cultures of the E-negative subset. The lytic potential was generated mainly in the population with low cell density, while the dense lymphocytes showed mostly proliferative responses. Part of the cells that lysed allogeneic PHA blasts lysed also autologous PHA blasts. Limiting dilution experiments are performed presently in order to establish whether cells reactive with autologous, but not allogeneic, PHA blasts are generated in MLC. Tumor promoters were shown to enhance the natural cytotoxicity of human lymphocytes. The effect is not mediated through induction of interferon production. The treatment of lymphocytes with phorbol esters reduces their recycling capacity; therefore, they show low natural killer potential in the 51Cr release assay. After a few hours, cells are activated for cytotoxicity, and, therefore, an elevated number of cells participate in the target lysis. The net effect is stronger, since high proportion of the first effector-target-interaction leads to lysis. The recognition of methylcholanthrene sarcoma cells by cytotoxic lymphocytes was studied with regard to the H-2 restriction phenomenon. An F1 tumor and its sublines selected for loss of oneof the parental alloantigen complexes were used. H-2 restriction of cytolytic T cells could have shown in vitro and also in graft neutralization assays in vivo. Passive transfer of immunity with lymphocyte subsets showed a difference between the requirement of T-cell subsets depending on the MHC relationship of the lymphocytes and the recipient. (SR)